This application is a continuation-in-part of application Ser. No. 533,128, filed Sept. 19, 1983, now abandoned.
This invention relates broadly to batteries of the type which comprise active or oxidizable metal anodes and cathode-electrolytes of the type containing certain materials capable of acting both as an electrolyte carrier, i.e., as a solvent for an electrolyte salt, and as the active cathode material for the battery. Such batteries are known in the art and are described by way of example in U.S. Pat. No. 4,328,289, issued May 4, 1982 to Zupancic, et al; U.S. Pat. No. 4,264,687, issued Apr. 28, 1981 to Dey, et al and U.S. Pat. No. 3,998,657, issued Dec. 21, 1976 to Auborn, et al.
Batteries of this type are desirable for many low rate applications such as powering heart pacemakers and other medical implanted devices, fire alarms, watches and calculators. The most common and well-known battery of this type is the lithium/thionyl chloride battery which will be described further hereinbelow with reference to the preferred embodiment of this invention.
These kinds of batteries also include a cathode current collector which is exposed to the cathode electrolyte as is the oxidizable active anode. This invention is specifically concerned with improved current collectors for such batteries.
Broadly speaking, the active oxidizable anode material for these batteries is usually preferably lithium metal. However, other oxidizable anode materials are used in these kinds of batteries and generally include the other alkali metals, such as sodium and potassium, etc., and alkaline earth metals such as calcium and alloys of these metals. The anode may typically be constructed of the oxidizable metal in contact with a suitable supporting metal grid. The grid for a lithium anode, for example, may be made of nickel, nickel alloys such as Monel, stainless steel, tantalum or platinum.
The cathode-electrolyte solvents known to be useful in these batteries, in addition to thionyl chloride, include sulphur dioxide and other fluid oxyhalides, nonmetallic oxides, halogens, non-metallic halides and mixtures thereof such as phosphorus oxychloride (POCl.sub.3) selenium chloride (SeCl.sub.2), sulphur trioxide (SO.sub.3), vanadium oxytrichloride (VOCl.sub.3), chromicoxychloride (Cr.sub.2 Cl.sub.2), sulphuric oxychloride (SO.sub.2 Cl.sub.2), nitrile chloride (NO.sub.2 Cl), nitrosyl chloride (NOCl), nitrogen dioxide (NO.sub.2), sulphur monochloride (S.sub.2 Cl.sub.2) bromine (Br.sub.2), chlorine (Cl.sub.2) and sulphur monobromide (S.sub.2 Br.sub.2). Solvents of this type can be used together with thionyl chloride (SOCl.sub.2) or separately. Other non-aqueous solvents may be included along with the aforementioned cathodic active materials such as organic solvents including propylene carbonate, acetonitrile, methyl formate, tetrahydrofuran and the like which have been generally used in non-aqueous high energy density lithium and lithium/SO.sub.2 cells.
Preferably, the electrolyte salt or salts included in the cathode-electrolyte as the solute thereof should provide an acceptable conductivity at the operating temperature of the environment within which the battery is to be used. Examples of electrolyte salts commonly used in various batteries of this type include alkali and alkaline earth metal halides, tetrahaloaluminates, tetrahaloborates, and soluble lithium salts such as LiiCl.sub.4, LiSbCl.sub.6, Li.sub.2 TiCl.sub.6, LiAlBr.sub.4, LiBCl.sub.4 and LiBF.sub.4.
As already indicated, the batteries of the type described herein also require a cathode current collector which includes a porous element of an inert conductive material in contact with the cathode-electrolyte. Preferably, the porous element of the current collector is a high-surface-area body of any particular shape and form required for the particular battery design contemplated. Porous carbon cathode elements are preferred. Satisfactory carbon cathode current collector elements of this type may be formed by pressing particulate carbon such as carbon black or acetylene black or graphite to a desired form. To impart a cohesive characteristic to such particulate pressed bodies, a suitable binder material may be added to the particulate. Suitable binder materials for this purpose include polytetrafluoroethylene, fluorinatedethylene propylene polymer, polyethylenetetrafluoroethylene, polychlorotrifluoroethylene, polyethylene chlorotrifluoroethylene and the like. Polytetrafluoroethylene is the preferred binder for carbon current collectors. The binder, when used, may be added in an amount between about two percent and about sixty percent by weight of the formed cathode current collector.
Although porous carbon or graphite such as Shawinigan carbon black (Shawinigan Products Co., Englewood Cliffs, N.J.) are generally the most preferred inert cathode current collector materials, other inert conductive materials such as nickel and stainless steel have been used as cathode current collectors in the types of cells with which this invention is concerned.
Batteries of the aforementioned type may also employ a suitable separator to prevent the reaction of anode and cathode materials when no electric current flows through the external circuit. Since the cathode material is not spontaneously reactive with the anode material, mechanical separators which only prevent contact between the two electrodes can be used. A wide variety of ceramic and plastic materials having small pore sizes are available for this purpose. Examples of such materials include: alumina, beryllia, magnesia, zirconia, titania, porcelain, porous glass, fritted glass, non-woven porous polytetrafluoroethylene and other fluoronated polymers, polypropylene and polyethylene. A preferred porous separator is the fiberglass cloth which is typically used as filter paper. Such cloth, known as "glass paper" may be obtained from Mead Corp., Specialty Paper Div., South Lee, Mass. 01260.
As already pointed out, the various aforementioned elements of the battery must be placed in operating relationship whereby both the anode and cathode current collector, although maintained in a spaced relationship, are exposed to the cathode-electrolyte. The separator, if used, is placed between the anode and the cathode current collector and is also exposed to the cathode-electrolyte.
The container for such a battery may be made of various metals such as iron, nickel or preferably stainless steel or it may be made of plastic coated metals, or other suitable materials. Insofar as the preferred form of this invention is concerned, the container will be of a metal such as stainless steel and will be adapted to function as the cathodic or positive terminal for the battery by being placed into direct contact with the cathode current collector.
Cells of the aforementioned type have been found to be susceptible to a thermal run-away reaction which tends to generate heat in the battery faster than the battery structure can dissipate it. In extreme situations this can result in partial melt-down of cell constituents such as the anode material, and has even been reported as causing minor explosions. In this connection reference may be made in U.S. Pat. No. 4,307,160 issued Dec. 22, 1981 to Shipman, et al.
It has been discovered that these disadvantages can be effectively controlled by introducing into the cell design unique features relating to the cathode current collector which effectively control the current capability of the battery, i.e., minimize it, thereby increasing the safety of the battery.
More specifically, for any given battery design such as one utilizing a carbon element for the current collector and a thionyl chloride containing cathode-electrolyte, which is the preferred embodiment of this invention, too large a surface area contact between the carbon element and its terminal or electrical contact, which in the case of the preferred embodiment is the battery case itself, fosters the aforementioned thermal runaway reaction. It is, therefore, desirable to control the surface area contact between the carbon element and the electrical terminal means, such as the battery case. The amount of this contact will, of course, depend on the current density desired and the particular design of the battery involved.
As a corollary to the above, the present invention provides, in the case of a battery designed to utilize the battery case as the cathodic or positive terminal, consistent contact between the carbon element and the case which is particularly important for constant reproducible electrical characteristics.